INTERNET DRAFT Mark Delany, Editor
Title: draft-delany-nullmx-00.txt Yahoo! Inc
Expires: 4 October 2005 5 April 2005
A NULL MX Resource Record means "I never accept email"
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Abstract
A common strategy of SMTP servers when deciding whether to accept an
email or not, is to ensure that the 2821.MailFrom contains a domain
willing to accept non-delivery email (aka bounces).
When the 2821.MailFrom domain has a DNS MX Resource Record (RR), it is
making an explicit statement that it is willing to accept
email. However, when the domain has just a DNS A (or AAAA) RR, there
is no such clarity as most hosts on the Internet advertise an A RR
regardless of whether they want to accept email or not.
The NULL MX RR formalizes the existing mechanism by which a domain
communicates that it will never accept email.
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Introduction
This document formally defines the "NULL MX" as a simple mechanism by
which a domain can indicate that it will never accept email.
SMTP clients have a prescribed sequence for resolving how to deliver
email to a domain. Section 5 of RFC2821 [RFC2821] covers this in
detail, but in essence the SMTP client first looks up a DNS MX RR and
if that is not found it falls back to looking up a DNS A RR.
However, many domains do not accept email, but do have A records. If
they have no MX records then senders will attempt to deliver mail to
those A records.
If there is no SMTP listener at that address then the message will be
attempted repeatedly for a long period before the sending MTA gives up
on delivery. This will delay notification to the sender in the case of
misdirected mail, and will consume resources at the sender.
If the domain has an SMTP listener at that address that rejects all
connections (for instance with a 554 response as a connection-opening
response) or has MX records pointing to such a listener then the
sender will be notified in a timely fashion, but resources (generating
a bounce) will still be consumed by the sender and it requires
additional services to be provided which wouldn't usually be needed.
These resource usage problems are exacerbated when large volumes of
email are sent using falsified email addresses in a domain which does
not accept email as its envelope sender, causing large numbers of
bounces to be generated and to consume large amounts of resources
(spool space and outgoing TCP sessions) at the sender of the bounces.
This document formally defines a NULL MX as an alternative which will
cause all mail delivery attempts to a domain to fail immediately,
without any reconfiguration of existing MTAs.
SMTP server benefits
Being able to detect domains that never accept email offers many
resource savings to an SMTP server. In the first instance, it can
choose to reject email during the SMTP conversation that does not
present a deliverable 2821.MailFrom domain.
In the second instance, if an SMTP server accepts an email, it can be
confident that an attempt to send a non-delivery email will likely be
answered by another SMTP server. This greatly helps to reduce
non-delivery queues. This contrasts greatly with the current situation
where a non-delivery email for, eg, www.example.net, will sit in the
queue for a full queue lifetime as SMTP connection attempts to
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www.example.net simply timeout.
Parallel Considerations
Clearly the perpetrators of abusive mail can adapt such that the "vast
class of email" that this mechanism helps identify, simply move over
to using 2821.MailFrom domains that have valid MX RRs.
Certainly this is true, but the direct benefits to the SMTP server still
apply. When an SMTP server queues a non-delivery email, the target
domain will accept the email or give a definitive rejection so the queue
entry will be removed promptly, thus keeping the queues short.
More importantly, there are parallel developments in Internet email,
particularly in sender authentication, that provide better mechanisms
for SMTP servers to authenticate email from those domains that do
legitimately accept email.
The NULL MX Resource Record
To indicate that a domain never accepts email, it advertises a
solitary MX RR with a RDATA section consisting of an arbitrary
preference number 0, and a dot terminated null string as the mail
exchanger domain, to denote that there exists no mail exchanger for a
domain.
The dot termination denotes that the null MX domain is considered to
be absolute, and not relative to the origin of the zone, the behavior
of dot termination and the formatting of this record is as described
in STD13 [STD13].
The interpretation of a NULL MX RR only applies when the domain as a
single MX RR. If a domain advertises multiple MX RRs, the
interpretation is unchanged from that defined by RFC2821.
The "I never send email" Corollary
A presumption of an SMTP server when presented with an "I never accept
email MX" might be to decline to accept such email as it knows that a
non-delivery will never be accepted.
SMTP servers MAY 550 reject mail sent by a MAIL FROM domain that has a
NULL MX record.
Security Considerations
SMTP mail is inherently insecure in that it is feasible for even
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fairly casual users to negotiate directly with SMTP servers. This
proposal is about eliminating one small section of SMTP insecurity.
It is claimed that there are legitimate cases where a domain may send
email but never wants to receive email. SMTP servers that reject mail
from domains that advertise a NULL MX risk losing email from those
domains.
Normative References
[STD13] Mockapetris, P., DOMAIN NAMES - CONCEPTS AND
FACILITIES", STD 13, November, 1987.
[RFC2821] Klesin, J., Editor, "Simple Mail Transfer Protocol",
RFC 2821, April 2001.
Acknowledgments
The editor wishes to thank Steve Atkins, James Lick, John Levine, der
Mouse, Daniel Quinlan and Suresh Ramasubramanian for their valuable
suggestions and constructive criticism.
Editor's Address
Mark Delany
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Email: mx0dot@yahoo.com
Please send comments to the editor at the above address. This feedback
email address will remain valid until this draft expires or is
replaced with a subsequent revision.
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RCS: $Id: draft-delany-nullmx-00.txt,v 1.2 2005/04/05 15:42:29 markd Exp $
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